$ dune rules <path-to-dune-file> -m -o Makefile
$ ls
Makefile ...

$ dune rules <path-to-dune-file> -m -o Makefile
Internal error, please report upstream including the contents of _build/log.
Description:
  ("Fdecl.get: not set", {})
Description:
  ("Fdecl.get: not set", {})
Description:
  ("Fdecl.get: not set", {})
Raised at file "src/stdune/code_error.ml", line 9, characters 30-62
Called from file "src/stdune/fdecl.ml" (inlined), line 29, characters 13-53
Called from file "src/stdune/path.ml", line 833, characters 26-60
Called from file "src/stdune/option.ml", line 9, characters 21-26
Called from file "src/stdune/option.ml" (inlined), line 21, characters 15-28
Called from file "bin/print_rules.ml", line 110, characters 12-44
Called from file "vendor/cmdliner/src/cmdliner_term.ml", line 25, characters
  19-24
Called from file "vendor/cmdliner/src/cmdliner.ml", line 146, characters 9-16
Called from file "vendor/cmdliner/src/cmdliner.ml", line 176, characters
  18-36
Called from file "vendor/cmdliner/src/cmdliner.ml", line 312, characters
  20-46
Called from file "bin/main.ml", line 261, characters 10-51

$ dune rules <path-to-dune-file> -m > Makefile
$ ls
Makefile ...